**Core Concept**
The composition of semen is a complex process involving the secretions of multiple glands in the male reproductive system. Semen is a mixture of sperm, seminal fluid, and other substances that provide the optimal environment for sperm transport and survival.

**Why the Correct Answer is Right**
The seminal vesicles are the primary source of the major part of semen, producing a clear, serous fluid that accounts for approximately 60% of the total volume of semen. This fluid is rich in fructose, which serves as an energy source for the sperm, and also contains prostaglandins, which help to stimulate uterine contractions and facilitate sperm transport. The seminal vesicles secrete this fluid in response to sympathetic nervous system stimulation.

**Why Each Wrong Option is Incorrect**
**Option A:** The prostate gland does contribute to semen composition, but it is responsible for only a small portion of the total volume. It secretes a milky fluid that helps to neutralize the acidity of the vagina and provides additional nutrients for the sperm.

**Option B:** The bulbourethral glands (Cowper's glands) secrete a clear, mucoid fluid that helps to lubricate the urethra and provide a conduit for sperm to pass through. However, this fluid accounts for only a small portion of the total volume of semen.

**Option C:** The epididymis is a tube-like structure that stores and transports sperm, but it does not contribute to the composition of semen.

**Clinical Pearl / High-Yield Fact**
The fructose content in seminal fluid is an important energy source for sperm, and its presence helps to differentiate normal semen from abnormal or pathological semen samples.

**Correct Answer: C. Seminal vesicles.